Of the three ways carriers might boost the capacity of the mobile networks we increasingly rely on, the one they will try in Geneva next month is probably the hardest.

Adding extra cells to their networks in order to reuse the same radio frequencies in more places just involves some extra base station hardware -- and permission from a lot of farmers and building owners to erect new antennas.

Using a more efficient radio protocol to shoehorn more bits per second into the same radio channels also only involves upgrades to base stations -- and, of course, to every subscriber's phone to make them compatible with the new protocol.

Both of those, though, are easy compared to the third option: finding some unused or underused radio spectrum that mobile networks can use to expand existing services or introduce new ones.

That's because, on top of the upgrades to base stations and phones, there's also the small matter of persuading the whole world's governments to agree on the change.

Network operators and phone manufacturers prefer to use the same frequency bands worldwide, as this allows subscribers to roam and manufacturers to benefit from economies of scale. But in the days before smartphones, each continent, and in some cases each country, allocated those frequency bands for different purposes -- radar, medical, broadcasting, aeronautical, military, satellite communications. These days, national governments make such decisions at the World Radio Conference, held in Geneva every three or four years by the International Telecommunication Union, a United Nations body. The next one, WRC-15, runs for four weeks from Nov. 2.

At WRC-15, governments have been asked to consider whether frequencies not being used for their declared purpose in most parts of the world could be reallocated to mobile broadband services.

Decisions now could result in new networks being built around 2020 by which time, the ITU estimates, the total spectrum requirement for mobile broadband could reach 1720MHz, compared to around 1000MHz today.

However, some Asian countries have identified as little as 51MHz of unused spectrum they are willing to release, nowhere near enough to satisfy network operators.

Meeting the operators' needs will mean finding a lot of underused bands. Candidates identified at the last WRC, in 2012, include the 2700-2900MHz band, currently used for airport and meteorological radars; the 3400-3700MHz used for radio location, and two bands used for satellite communications, 3400-4200MHz and 4500-4800MHz.

These bands have the advantage that their use is already harmonized worldwide. However, with the exception of the 2700MHz-2900MHz band, which sits next to one used for 4G LTE service, they are at far higher frequencies than those used for mobile broadband today, and would require significant changes to radio hardware and network design.

More attractive to operators, at least in the short term, is the so-called L-band, between 1.4GHz and 1.5GHz, already used for mobile broadband in Japan. Situated between the main bands used for the second generation of mobile communications, this band's lower frequencies tend to travel further and penetrate buildings better than the others under consideration, making it easier to roll out coverage to wide areas. There's a large contiguous block of spectrum potentially available, too, at least 40MHz and perhaps as much as 80MHz. Such large blocks are more valuable than the same amount of spectrum cut into thinner slices, as more advanced radio protocols such as LTE can be used.

But with less than a week to go before WRC-15 begins, the GSMA, a lobby group representing almost all the world's mobile operators, said that there is still significant opposition to the L-band proposal in Asia, where just 12 countries are backing a Japanese proposal to make 40MHz available. Russia is also against the proposal, which otherwise has strong support, particularly in the U.S. and Europe. The GMSA published a study claiming that those Asian holdouts could add see economic benefits of up to US$9 billion from the technology if they were to add their support.

The GSMA is more optimistic about prospects for reallocating spectrum under 700MHz, historically used for TV broadcasting, it said Thursday.

But it's at the other end of the radio spectrum that the most radical proposals are to be found: The Next Generation Mobile Network Alliance, representing some of the world's largest mobile networks, wants the WRC-15 to allocate frequencies between 6GHz and 100GHz for mobile broadband use. The ITU hadn't planned to discuss that spectrum until the next World Radiocommunications Conference, in 2019.

Copyright 2017 IDG Communications. ABN 14 001 592 650. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of IDG Communications is prohibited.